HTTP/2 Write Compression Support

Motivation:
The HTTP/2 codec does not supply an interface which facilitates doing data write compression.
Compressing data is anticipated to be a common use case and this feature is thus anticipated to be useful.

Modifications:
-Restructure the AbstractHttp2ConnectionHandler class to be able to extend write behavior before the outbound flow control gets the data
-Add classes which override the newly restructured interfaces to provide an extensible compression interface
-Add unit tests to test the new capability

Result:
HTTP/2 codec provides an interface (and default implementation) for data write compression.
Correct.  There are no intentional changes...just restructuring to allow easier extension.  I will point out stuff that changed because of the different interfaces.  Also look at the extra `write*` interfaces in `Http2OutboundFrameProducer.java` were to support `AbstractHttp2ConnectionHandler` functionality.